It does seem like a lot of trouble, doesn't it?
If you lived in Milwaukee, where I live, the
answer would be more obvious. Winters here are
very cold, and it is hard for many kinds of birds
to find food here in the winter because the ground
is covered, there are few insects moving around,
and there is little plant food around. A lot of
birds that live in the northern half of the planet
(the northern hemisphere) spend months in the
southern hemisphere.

When we are having
our winter, it is summer in the southern part of
the world. Nearest the equator, there really is
no summer or winter. Closer to the poles, the
seasons get more and more extreme. This is
because of the angle of the sun as it hits the
Earth, but that's a whole new topic.

Many
birds migrate from where conditions are bad to
where they are good. When things get good back
here in the northern hemisphere, these birds fly
back to take advantage of all of the food that's
around in the spring. There is even a species of
bird that spends half the year in the North Pole
and half the year in the South Pole. Now that's a
long trip.

If you live in a place like
Santa Barbara, CA, the "winter months" don't seem
that bad. Some birds will just stay year-round.
Even here in Wisconsin, we have year-round
resident birds. But many birds can do better by
flying south where there's more food.

Here's some "food for thought." How do
you think birds find their way on these long
journeys? Do you think they are born knowing the
way, or do they have to learn it? Why don't more
mammals migrate?

Answer 2:

During the summer, the climate warms up, and the
parts of the Earth that are comfortable for a
given species move poleward. During the winter,
climate gets cold again, and climes that are
comfortable move toward the equator. Of course,
while winter is happening in the Northern
Hemisphere, summer is happening in the Southern
Hemisphere, and vice-versa. Birds simply move to
parts of the world that they can live in in
response to the seasons.